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BMW D190 Riser/Exhaust Elbow Design

Tried posting on the BMW Marine Diesel forum but it is pretty much dead so I will try here. I just bought the engine but missing the riser after the turbo. Looked at replacements but the design is confusing to this "closed loop cooling" noobie. In the picture the flange end of the pipe bolts to the turbo. The raw cooling water from the heat exchanger is injected just after the turbo. When the engine is shut down, what prevents any water in the raw water tube from backflowing into the turbo? The stock riser is the same. I see there is a "nub" at the bottom of the pipe directly under the raw water inlet. Possibly it is a valve held shut by the exhaust pressure which then opens and drains when the engine shuts down?

Thanks, Friz
 

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  • Riser, SS.jpg
    Riser, SS.jpg
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  • Riser, Original.JPG
    Riser, Original.JPG
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  • Riser Mounted on Engine 2.jpg
    Riser Mounted on Engine 2.jpg
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Thanks anyhow. Have come up on the learning curve and now understand "wet risers" are double walled, pipe in pipe. My concern about saltwater draining into the turbo has been explained away.
 
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